The invention relates to the field of electronic reproduction technology and pertains to a method of setting the line width of recorded image lines in an exposer (output scanner, output recorder) for recording printing originals, such as printing plates.
In electronic reproduction technology, recording devices are used to expose printing originals and printing forms. For this purpose, a laser beam is, for example, produced by a laser diode, shaped by an optical device, and focused onto the recording material. The laser beam is deflected over the recording material, point by point, and line by line, by a deflection system. There are also recording devices which, in order to increase the exposure speed, produce a bundle of laser beams, for example, using a separate laser diode for each laser beam, and expose a plurality of image lines of the printing form simultaneously each time they sweep over the recording material. The printing forms can be exposed on film material to produce color separation films that can subsequently be used for producing printing plates with a photographic recopying method. Instead, the printing plates themselves can also be exposed in a plate exposer or directly in a digital press. The recording material can be located on a drum (external drum exposer), in a cylindrical hollow (internal drum exposer) or on a flat surface (flat bed exposer).
In the case of an external drum exposer, the material to be exposed, in the form of films or printing plates, is mounted on a rotatably mounted drum. While the drum rotates, an exposure head is moved axially along the drum at a relatively short distance. The exposure head focuses one or more laser beams onto the drum surface. The beams sweep over the drum surface in the form of a narrow helical line. In this way, during each revolution of the drum, one or more image lines are exposed on the recording material.
In the case of an internal drum exposer, the material to be exposed is mounted on the internal surface of a partly open hollow cylinder and is exposed with a laser beam that is aimed along the cylinder axis onto a deflection device that reflects the laser beam at right angles onto the material. The deflection device, a prism or a mirror, is rotated during operation and is additionally moved slowly in the direction of the cylinder axis, so that the deflected laser beam makes circular or helical image lines on the material, in order to scan the entire surface of the material little by little.
Flat bed exposers operate predominantly with a rapidly rotating polygonal mirror, whose mirror surfaces deflect the laser beam transversely over the recording material, while at the same time the recording material is moved at right angles to the deflection direction of the laser beam. In this way, exposure is carried out image line by image line. Since the length of the light path changes during the movement of the laser beam over the recording material, complicated imaging optics are required, which compensate for size changes thereby induced in the exposure point.
As compared with flat bed exposers, internal drum exposers have the advantage that the optical paths are shorter, so that less expenditure for vibration damping is required. In addition, the length of the light path is constant, so that it is possible to manage with simpler imaging optics. Because of the few and rapidly moving parts, high exposure speeds are achieved with internal drum exposers. Because of the construction, when using an external drum exposer, the achieved exposure speeds are not so high, since a large mass has to be moved and the rotational speed of the drum is limited by the centrifugal forces which, at too high a rotational speed, would separate the recording material from the drum. The advantage with the external drum exposer is that the light path from the laser to exposure point is very short. This permits a configuration of the imaging optics with which a very sharp beam profile is produced. Because of the longer light path, this is not possible in an internal drum exposer, so that the beam profile has a distribution of the power density from the center to the edge of the laser beam in the form of a Gauss bell curve, for example.
During the exposure of printing originals, the laser beam is not modulated with a continuously varying signal, but is switched on and off on the basis of a binary image signal which can assume only two values. The reason for this is that the printing originals are screened, that is to say different tonal values in the color separations are represented by halftone dots whose size is varied in accordance with the tonal values. During the exposure of the printing originals, the half tone dots are composed of many small exposure points, for example, with a resolution of 1,000 exposure points/cm. In order to achieve this resolution, the image lines must have a spacing of a=10 xcexcm, for example. Whether the diameter d of the exposure points in this example must likewise be set to d=10 xcexcm depends on the beam profile of the laser beam and on the exposure characteristics of the recording material. For the following explanations, the diameter d will be defined as 50% diameter that results from the radius at which the power density of the laser beam has fallen to 50% of its maximum value at the center of the beam.
If there is a sharp beam profile, such as in the case of an external drum exposer, that is to say there is a uniform distribution of the power density over the area of the exposure point, the 100% diameter of the exposure point is set to the dimension of the line spacing a or somewhat larger. This is because adjacent image lines adjoin one another exactly or overlap somewhat, which ensures that no unexposed areas are produced between adjacent image lines, which would be disruptively visible in the finished recording of the printing original. If, as in an internal drum exposer for example, the beam profile follows a gaussian curve, the 50% diameter d of the exposure point is preferably set such that it corresponds to the image line spacing a, that is to say, in the example of a resolution of 1,000 exposure points/cm, the line spacing a=10 xcexcm and the 50% diameter is likewise d=10 xcexcm. This basic setting of the diameter of the exposure point has been tried and tested in practice for films and printing plates with a photosensitive layer of silver halide, which generally have a light sensitivity curve with a pronounced exposure threshold. If the incident energy density exceeds the exposure threshold, the material is exposed, and at an energy density lying below this, it remains unexposed. The basic setting of the line width in an exposer is optimized during daily operation by being readjusted in fine steps by changing the laser power. In this way, varying characteristics of the recording material and of the developing scheme are compensated for. For this purpose, test exposures are carried out with various line widths and are measured. The parameters that result in the best line connection between adjacent image lines are then set in the exposer.
For some time, offset printing plates have been available which are built up from a carrier material and a photopolymer layer applied thereto and which behave differently than a typical photosensitive layer based on silver halide. FIG. 1 shows in qualitative terms a typical exposure curve for a photopolymer printing plate. Plotted on the horizontal axis is the energy density H with which the printing plate is exposed, and the effect W of the exposure is plotted on the vertical axis. When a first exposure threshold M1 is exceeded, polymerization is triggered in the photopolymer layer, and crosslinks the molecules. As a result, the exposed areas become resistant to the developer solution, with which the unexposed areas are washed out in a subsequent process. However, the exposed areas are not yet resistant enough against the wear in the press if they have been exposed only with an energy density that is slightly above the first exposure threshold M1. Therefore, exposed and developed printing plates do not last long enough to be able to print large editions. In order to consolidate the exposed areas in such a way that they become resistant to the wearing processes in the press, they must be exposed relatively far beyond the first exposure threshold M1. Beginning at a second exposure threshold M2, which may be considerably higher than the first exposure threshold M1, the consolidation of the polymer has progressed to such an extent that no substantial further consolidation is achieved with even more intense exposure. In the area of the second exposure threshold M2 and beyond this, the energy density H used for the exposure can be selected within wide limits, for example, in order to set the line width b optimally by changing the laser power. In this case, however, the known methods for the optimum setting of the diameter d of the exposure point or the line width can no longer be applied to a beam profile having a gaussian curve.
There is also the possibility of exposing the photopolymer printing plate in the exposer only with an energy density H corresponding to the first exposure threshold M1 and, following the development, of carrying out a reexposure with UV light in order thereby to reharden the polymer structures that have not been washed out. However, this mode of operation entails additional expenditure on time and labor and is therefore disadvantageous.
It is accordingly an object of the invention to provide a method of setting the line width of recorded image lines of a laser beam focused on an exposure point in an exposer for recording printing originals on a recording material, which overcomes the above-mentioned disadvantages of the prior art methods of this general type.
In particular, an object of the invention is to provide a method with which, for a photopolymer recording material and a given beam profile of the laser beam, the line width can be set optimally in an exposer, so that no underexposed areas are produced between adjacent image lines. In addition, the exposure is carried out in one operation with a sufficiently high energy density, so that no reexposure of the recording medium is required in order to consolidate the polymer structures.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of setting a line width of recorded image lines from a laser beam, which is focused on an exposure point in an exposer for recording printing originals on a recording material, and which has an exposure speed, while using a predefined image line spacing. The method includes steps of: setting a diameter of the exposure point to be smaller than the line spacing and providing an area of the exposure point with a power density having a non-uniform distribution; providing the recording material with surface characteristics and a first exposure threshold such that when the first exposure threshold is exceeded, the surface characteristics of the recording medium change; and in the exposer, exposing the recording material with an energy density that is substantially higher than the first exposure threshold.
In accordance with an added feature of the invention, the recording material includes a photopolymer layer.
In accordance with an additional feature of the invention, the energy density, which is used during the step of exposing the recording material, is near a second exposure threshold that ensures consolidation of the photopolymer layer.
In accordance with another feature of the invention, the exposer is an internal drum exposer.
In accordance with a further feature of the invention, the method includes determining the line width of the recorded image lines from the distribution of the power density over the area of the exposure point by integrating a time variation of a power density that results from the exposure speed of the laser beam.
In accordance with a further added feature of the invention, when performing the step of setting the diameter of the exposure point, the diameter of the exposure point is set such that the line width is substantially equal to the line spacing.
In accordance with a further additional feature of the invention, the diameter of the exposure point is given by a radius at which the power density has fallen to 50% of a maximum value of the power density.
In accordance with yet an added feature of the invention, the line width is set by changing a power of the laser beam.
In accordance with yet an additional feature of the invention, when performing the step of setting the line width, the line width is set to be substantially equal to the line spacing.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of setting the image line width in an exposer, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.